Nitride semiconductor laser diode

ABSTRACT

A nitride semiconductor laser diode comprises a substrate; an n-side nitride semiconductor layer containing an n-type impurity and disposed on the substrate; an active layer having a light emitting layer including In x Al y Ga 1−x−y N (0&lt;x&lt;1, 0≦y&lt;1, and 0&lt;x+y&lt;1) and disposed on the n-side nitride semiconductor layer; and a p-side nitride semiconductor layer containing a p-type impurity and disposed on the active layer. The lasing wavelength of the nitride semiconductor laser diode is 500 nm or greater. A concentration distribution of the p-type impurity in a depth direction from the light emitting layer toward the surface of the p-side nitride semiconductor layer has a local maximum with the concentration of the p-type impurity of 5×10 18  cm −3  or greater in a range within 300 nm from the top surface of the light emitting layer which is closest to the p-side nitride semiconductor layer, and after passing the local maximum, the concentration of the p-type impurity is not less than 6×10 17  cm −3  in the range within 300 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

The application is a Continuation Application of U.S. Ser. No.13/387,855, filed Apr. 10, 2012, which is the U.S. National Phase ofPCT/JP2010/062527, filed Jul. 26, 2010, which claims priority toJapanese Patent Application No. 2009-179810, filed Jul. 31, 2009. Eachof these applications is incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor laser diodeusing a nitride semiconductor represented byAl_(x)In_(y)Ga_(1−x−y)N(0≦x≦1, 0≦y≦1, 0≦x+y≦1).

2. Description of Related Art

A semiconductor laser diode using a compound semiconductor is widelyused in applications such as optical disc systems capable of recordingand/or reproducing large volume, high-density information. Meanwhile, anew application for semiconductor laser diodes which will provide a fullcolor display with a combination of blue, green, and red semiconductorlasers is expected to be realized.

Among blue, green, and red of the three principal colors of light, blueand red semiconductor laser diodes are already in practical use withemploying Group III-V compound semiconductors such as InAlGaN andAlInGaP. In contrast, as for a green laser, a laser device capable ofemitting a green beam has been developed by converting wavelength usingsecond harmonic generation (SHG). However, a direct green laser diodecapable of directly emitting a green beam has not yet been put intopractical use.

As for a semiconductor laser diode capable of directly emitting greenlight, a laser diode using a Group II-VI compound semiconductor wasreported around 1993, but because of its poor reliability at highcurrent condition, its practical use was not realized. For this reason,in recent years realization of a semiconductor laser diode capable ofdirectly emitting green light with a use of a Group III-V nitridesemiconductor is expected.

As for a Group III-V nitride semiconductor, a semiconductor laser diodeusing a light emitting layer made of In_(x)Al_(y)Ga_(1−x−y)N (0<x,0≦y<1, 0<x+y<1), particularly made of InGaN, (hereinafter referredsimply as “InGaN light emitting layer”) and capable of emittingultraviolet to blue light is already in practical use (Patent Reference1 etc.). When the In content in the InGaN light emitting layer isincreased, the band gap becomes smaller and the emission wavelengthbecomes greater and emission of green light becomes possible. However,in the case where the InGaN light emitting layer is grown by vapor phaseepitaxy, the lattice mismatch with respect to GaN layer which is anunderlayer thereof, becomes greater with increase of In content, andInGaN layer itself becomes chemically unstable and phase separationtends to occur. For this reason, the realization of a semiconductorlaser capable of emitting green light, by way of increasing the Incontent in the InGaN light emitting layer is not easy.

In Patent Reference 2, a semiconductor laser diode capable of emittinglight in a wide range of wavelengths including green is proposed, inwhich InGaNP, obtained by a part of Group V element in InGaN substitutedwith P is used as the light emitting layer. According to PatentReference 2, using InGaNP as the light emitting layer enables P, whichis a Group V element having less volatile than N, to combine with In,thus segregation of In can be prevented. In addition, bowing effect ofband gap due to the change in the content of P is large, so that thecontent of In necessary to obtain a desired emission wavelength can bereduced compared to that in a conventional InGaN light emitting layer.Also, Patent Reference 3 proposes an addition of an impurity such as Mg,Be, C, or Si to InGaNP light emitting layer to prevent separation ofcrystalline system in the InGaNP light emitting layer.

In contrast, recently, the inventors of the present invention improvethe crystal quality of the InGaN light emitting layer having a highcontent of In by optimizing its growth condition and succeeded inproducing continuous lasing at room temperature up to 515 nm in asemiconductor laser diode using an InGaN light emitting layer, andreported in Non-patent Reference 1. The green semiconductor laserexhibited output power of 5 mW at 25° C. and estimated operating life atroom temperature was 5000 hr or greater.

Patent Reference 1: WO 2002-05399

Patent Reference 2: JP 2002-26459A

Patent Reference 3: JP 2002-84040A

Non-patent Document 1: T. Miyoshi et. al., “510-515 nm InGaN-Based GreenLaser Diode on c-Plane GaN substrate,” Applied Physics Express 2(2009),062201.

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

However, the green semiconductor laser diode using the InGaN lightemitting layer described above did not yet have sufficient lifecharacteristics. As shown in Non-patent Reference 1, a 510-513 nmwavelength green semiconductor laser diode exhibits a stable drivingcurrent to 500 hours in a life test at room temperature operation, andthe operating life at room temperature which is estimated from the testwas 5000 hours or greater (see Non-patent Reference 1, FIG. 8). However,in a high temperature operation at 60° C. performed as accelerated test,sharp deterioration was observed from immediately after startingoperation, and the operating life in high temperature operation was onlyseveral tens of hours.

According to the findings by the inventors of the present invention, thelife characteristics of the semiconductor laser diode using InGaN lightemitting layer is strongly affected by the wavelength, and the lifecharacteristics sharply decreases when the wavelength is 500 nm orgreater. It is conventionally known that with an increase in the Incontent in InGaN light emitting layer, lattice mismatch with the GaNlayer which is the under layer increases and dislocations are generatedat the interface between the InGaN layer and the GaN layer. It has alsobeen known that with a increase in dislocation density in a laser diodeelement, the life characteristics decreases. However, the cause of thedecrease in the life characteristics with the increase in thedislocation density is thought to be that the dislocations act asnon-radiative dislocation centers. For this reason, conventionally, inorder to prevent decrease in life characteristics with increasingemission wavelength, efforts have been exerted to reduce the generationof dislocations.

For example, reducing the thickness of the InGaN light emitting layerfacilitates reduction of distortion caused by lattice mismatch with theGaN layer which is the underlayer thereof, and generation ofdislocations can be prevented to some degree. However, when the InGaNlight emitting layer is made thin, the efficiency of carrier containmentin the InGaN light emitting layer drops, which causes an increase in thethreshold current.

Meanwhile, in Patent Reference 2 and Patent Reference 3, InGaNP orInGaNAs is used for the light emitting layer, instead of InGaN, and thusa desired wavelength with a small In content is obtained. With thisarrangement, prevention of segregation of In and reduction of latticemismatch are aimed (Patent Reference 2, paragraph [0041]). However,introduction of As or P in the InGaN light emitting layer tends to leadto deterioration of the crystal quality of light emitting layer.Further, in a light emitting layer containing As or P, segregation of Asor P tends to occur in the vicinity of dislocations which leads todeterioration of the crystal quality of the light emitting layer, sothat reduction in the light emitting efficiency and increase inthreshold current tend to result (Patent Reference 3, paragraph [0012]).

Accordingly, an object of the present invention is, in a nitridesemiconductor laser diode using an InGaN light emitting layer and havinglasing wavelength of 500 nm or greater, suppressing deterioration oflife characteristics associated with a long lasing wavelength whilefavorably maintaining confinement of carriers in the InGaN lightemitting layer and its crystal quality to realize a long-wavelengthnitride semiconductor laser diode excellent in light emitting efficiencyand life characteristics.

Means to Solve the Problem

The inventors of the present invention have diligently studied anddiscovered that a rapid decline in life characteristics in a nitridesemiconductor laser diode of lasing wavelength 500 nm or greater is notcaused by the dislocations themselves, but due to the effect of thedislocations, activation of the p-type impurity is inhibited by theeffect of dislocations and the resulting insufficient supply of holes tothe InGaN light emitting layer causes the decline in the lifecharacteristics. According to the present invention, while increasingthe In content and allowing the generation of dislocations in the activelayer, the concentration of a p-type impurity within a predetermineddistance from the InGaN light emitting layer is controlled in apredetermined range. Thus, lasing at a long wavelength of 500 nm orgreater is realized and the life characteristics is dramaticallyimproved.

That is, a nitride semiconductor laser diode according to the presentinvention includes a substrate, an n-side nitride semiconductor layercontaining an n-type impurity and disposed on the substrate, an activelayer having a light emitting layer including In_(x)Al_(y)Ga_(1−x−y)N(0<x<1, 0≦y<1, 0<x+y<1) and disposed on the n-side nitride semiconductorlayer, and a p-side nitride semiconductor layer containing a p-typeimpurity and disposed on the active layer. The lasing wavelength of thenitride semiconductor laser diode is 500 nm or greater. Dislocationsoriginated in the active layer penetrate through the p-side nitridesemiconductor layer with a dislocation density of 1×10⁶ cm⁻² or greater,and a concentration distribution of the p-type impurity in a depthdirection, from the light emitting layer toward the surface of thep-side nitride semiconductor layer, has a local maximum with theconcentration of the p-type impurity of 5×10¹⁸ cm⁻³ or greater in arange within 300 nm from the top surface of the light emitting layerwhich is the closest layer to the p-side nitride semiconductor layer,and after passing the local maximum, the p-type concentration is notless than 6×10¹⁷ cm⁻³ in the range within 300 nm.

Here, the term “local maximum” in the concentration distribution of ap-type impurity in a depth direction refers to a place where theconcentration of the p-type impurity changes from increasing todecreasing. Conversely, the term “local minimum” refers to a place wherethe concentration of the p-type impurity changes from decreasing toincreasing. Here, the expression “increasing” or “decreasing” includesnot only the cases where the concentration of p-type impurity withrespect to the depth changes in a continuous manner but also the caseswhere it changes in a discontinuous manner.

In the conventional short wavelength nitride semiconductor laser diode,when the concentration of p-type impurity in the vicinity of the lightemitting layer becomes greater, internal loss due to optical absorptionincreases, which may cause an increase in threshold current and decreasein slope efficiency. For this reason, it has been known that the dopingamount of p-type impurity is better to be small in the vicinity of thelight emitting element. Also, the dislocations act as non-radiativerecombination centers, so that it has been a common practice to reducegeneration of dislocations in the light emitting layer to a minimum. Onthe contrary, in the present invention, the concentration of p-typeimpurity in the vicinity of the light emitting layer is increasedcompared to that in conventional methods and controlled in the rangedescribed above, while allowing the generation of dislocations in theactive layer of a long wavelength nitride semiconductor laser diode.With this, life characteristics of nitride semiconductor laser diode inlong wavelength region of 500 nm can be exponentially improved, and itsestimated operating life at high-temperature operation, which has beenconventionally only several tens of hours, can be extended at once to apractical level of more than several thousand hours. Such a dramaticimprovement in the life characteristics by controlling the Mgconcentration has not been observed in conventional short wavelengthnitride semiconductor lasers and is a specific phenomenon to the longwavelength nitride semiconductor laser with lasing wavelength of 500 nmor greater. The reason for the phenomenon is not necessarily clear, butis considered below.

In the present invention, the dislocations associated with an increasein the In content in the InGaN light emitting layer is intentionallyleft in situ, thus enabling prevention of occurrence of problems such asan insufficient carrier confinement due to decrease in the thickness ofthe InGaN light emitting layer, and deterioration of crystal quality dueto the introduction of other Group V elements. On the other hand, due tothe increase of the In content for obtaining a lasing wavelength of 500nm or greater, dislocations originated in the active layer occur. Thus,the dislocation density of a nitride semiconductor layer located abovethe light emitting layer increases, where the dislocation density in thep-side nitride semiconductor layer is at least 1×10⁶ cm⁻². The degree ofcrystal quality of the nitride semiconductor layer decreases in thevicinity of dislocations, so that generation of holes by the p-typeimpurity becomes insufficient with increase of dislocation density,leading to insufficient supply of holes to the InGaN light emittinglayer. As a result, the electrons diffuse to the p-side of the InGaNlight emitting layer and cause non-radiative recombination that sharplyreduces life characteristics. The sharp reduction in the lifecharacteristics is not simply due to the dislocation density in thep-side nitride semiconductor layer but also depends on the origin of thedislocations. When the In content in the InGaN layer is increased toobtain a long lasing wavelength of 500 nm or greater, dislocations occurfrom the active layer. The dislocations originating from the activelayer affect the crystal quality of the p-side nitride semiconductorlayer, which leads to insufficient supply of holes to the InGaN lightemitting layer, that sharply reduces life characteristics. Such aphenomenon is not observed with the dislocations propagating from thesubstrate. For example, even in a short wavelength nitride semiconductorlaser diode with a low In content in the InGaN light emitting layer, ifthe dislocation density in the substrate is high, the dislocationdensity in the p-side nitride semiconductor layer may be 1×10⁶ cm⁻² orgreater. However, in such cases, the sharp change in the lifecharacteristics due to the concentration of p-type impurity exhibited inthe nitride semiconductor laser diode with lasing wavelength of 500 nmor greater is not observed. From this, it is thought that thedislocations originating from the active layer and the dislocationspropagated from the substrate exert different effect on the p-sidenitride semiconductor layer, and therefore the critical dependency onthe concentration of p-type impurity which is characteristic of longwavelength as described above occurs. A large fraction of thedislocations originating in the active layer has its origin in the lightemitting layer.

For this reason, in the present invention, the concentrationdistribution of p-type impurity in a depth direction is controlled sothat in a range within 300 nm from the top surface of the light emittinglayer which is the closest layer to the p-side nitride semiconductorlayer, the concentration of the p-type impurity has a local maximum of5×10¹⁸ cm⁻³ and not less than 6×10¹⁷cm⁻³ after passing the local maximumand within the 300 nm range. Within the distance of 300 nm, which allowsefficient supply of holes to the InGaN light emitting layer, theconcentration of the p-type impurity is once increased to 5×10¹⁸ cm⁻³ orgreater, and decreased to not less than 6×10¹⁷ cm⁻³. Thus, supply ofholes to the InGaN light emitting layer is secured and occurrence ofnon-radiative recombination in the p-side of the InGaN light emittinglayer is prevented, and accordingly, exponential improvement in the lifecharacteristics can be achieved. Moreover, because light from the lightemitting layer is concentrated in the region of about 300 nm from thetop surface of the light emitting layer, the absorption of the emissionby the p-type impurity can be suppressed while securing the supply ofholes to the light emitting layer, by controlling the concentration ofthe p-type impurity so that the concentration once increases to 5×10¹⁸cm⁻³ or greater and decreases to not less than 6×10¹⁷ cm⁻³.

Effect of the Invention

As described above, according to the present invention, in a nitridesemiconductor laser diode of lasing wavelength 500 nm or greater,occurrence of the dislocations originating from the light emitting layerwith an increase in the In content is intentionally left in situ. Thus,while favorably maintaining the carrier confinement in the InGaN lightemitting layer and the crystal quality thereof, the concentration ofp-type impurity present within a predetermined distance from the InGaNlight emitting layer is controlled in a predetermined range, and thusexponential improvement in the life characteristics can be achieved.Accordingly, a long wavelength nitride semiconductor laser diode withexcellent light emitting efficiency and life properties can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of anitride semiconductor laser diode according to the present invention.

FIG. 2 is a partially enlarged schematic cross-sectional view of theactive layer and the p-side nitride semiconductor layer of the nitridesemiconductor laser diode shown in FIG. 1.

FIG. 3 is a diagram illustrating a relationship between the lasingwavelength and the estimated operating life of a nitride semiconductorlaser diode.

FIG. 4 is a cross-sectional view schematically illustrating generationof dislocations in an active layer.

FIG. 5 is a diagram illustrating a relationship between the lasingwavelength and the dislocation density of a nitride semiconductor laserdiode.

FIG. 6 is a diagram illustrating the result of a high-temperature lifetest of a nitride semiconductor laser diode of a comparative example.

FIG. 7 shows I-V characteristic in the initial stage and after applyingcurrent of a nitride semiconductor laser diode of a comparative example.

FIG. 8 is a diagram illustrating the concentration distribution of ap-type impurity in a depth direction in a nitride semiconductor laserdiode of a comparative example.

FIG. 9 is a diagram illustrating the concentration distribution of ap-type impurity in a depth direction in a nitride semiconductor laserdiode of an example.

FIG. 10 is a diagram illustrating the result of a high-temperature lifetest of a nitride semiconductor laser diode of an example.

FIG. 11A is a diagram illustrating a relationship between the localmaximum value in the concentration of a p-type impurity and theestimated operating life of a nitride semiconductor laser diode.

FIG. 11B is a diagram illustrating a relationship between the localminimum value in the concentration of a p-type impurity and theestimated operating life of a nitride semiconductor laser diode.

FIG. 12 is a schematic cross-sectional view illustrating a layerstructure of an n-side nitride semiconductor layer.

FIG. 13 is a schematic cross-sectional view illustrating a ridgestructure disposed on a p-side nitride semiconductor layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings. Each drawing is a schematicdiagram and the arrangement, dimension, ratio, shape, and the like, maybe altered.

FIG. 1 is a schematic cross-sectional view showing an example of anitride semiconductor laser diode according to the present invention. Onthe substrate 2 made of a nitride semiconductor, an n-side nitridesemiconductor layer 4 containing an n-type impurity such as Si, anactive layer 6, and a p-side nitride semiconductor layer 8 containing ap-type impurity such as Mg, are stacked, and a ridge 36 constituting awaveguide is formed in a part of the p-side nitride semiconductor layer8. The portion around the ridge 36 is covered with an embedded layer 46and a protective film 48 is further disposed thereon. A p-side electrode38 is disposed on a part of the p-side nitride semiconductor layer 8which is exposed from the top of the ridge 36, and further, a p-side padelectrode 40 is disposed to cover the ridge 36 while being in contactwith the p-side electrode 38. Meanwhile, an n-side electrode 42 isdisposed on the back surface of the substrate 2 made of a nitridesemiconductor.

FIG. 2 is a partially enlarged schematic cross-sectional view of theactive layer 6 and the p-side nitride semiconductor layer 8 of thenitride semiconductor laser diode shown in FIG. 1. The active layer 6has a multiquantum well structure in which a barrier layer 22 a, 22 b,22 c made of InGaN or GaN and a well layer 24 a, 24 b made of InGaN arealternately stacked, where the InGaN well layers 24 a, 24 b serve as thelight emitting layer. Also, as the p-side nitride semiconductor layer 8,an Al-containing nitride semiconductor layer 26 (first p-type nitridesemiconductor layer), a p-side optical guide layer 28 a, 28 b (secondp-type nitride semiconductor layer), a p-side cladding layer 32 (thirdp-type nitride semiconductor layer), and a p-side contact layer 34 arestacked in this order from the side closer to the active layer 6. Thestripe structure having a layered structure shown in FIG. 1 and FIG. 2constitutes the resonator cabity and thus provides an edge-emitting typelaser diode.

The lasing wavelength of the nitride semiconductor laser diode shown inFIG. 1 can be selected variously by changing the In content in the InGaNlayers 24 a, 24 b, in which, a longer lasing wavelength can be obtainedwith increasing the In content. However, the life characteristics ofthis nitride semiconductor laser shows strong dependency to its lasingwavelength, and the operating life decreases rapidly at an lasingwavelength beyond 500 nm. In FIG. 3, as for a nitride semiconductorlaser diode having a structure as shown in FIG. 1, the estimatedoperating life is plotted versus the lasing wavelength, showing that theestimated operating life sharply decreases with the lasing wavelengthapproaching to 500 nm, and lasts only several tens of hours at awavelength of 500 nm or greater.

It is conventionally known that with an increasing in the In content inthe InGaN well layers 24 a, 24 b. the lattice mismatch with the barrierlayers 22 a, 22 b made of InGaN or GaN, which are the respectiveunderlayers thereof, increases, leading to generation of newdislocations in the active layer 6. According to the research conductedby the inventors of the present invention, in an active layer 6 with alasing wavelength exceeding 500 nm, as schematically shown in FIG. 4, anumber of dislocations were generated at the interface between the InGaNwell layer 24 a and InGaN barrier layer 22 a and the interface betweenthe InGaN well layer 24 b and the GaN barrier layer 22 b. If the Incontent is increased to obtain a longer lasing wavelength, thegeneration of dislocations also increases accordingly. FIG. 5 is adiagram illustrating a relationship between the lasing wavelength andthe dislocation density of the p-side nitride semiconductor layer 8, ina nitride semiconductor laser diode in which the dislocation density inthe substrate 2 is about 5×10⁵ cm⁻². In this case, the dislocationscontained in the substrate is about 5×10⁵ cm⁻², so that the valueobtained by subtracting 5×10⁵ from the value of dislocation densityshown in FIG. 5 is the dislocations newly generated in the active layer6. As shown in FIG. 5, at the lasing wavelength of about 480 nm,generation of the dislocations starts in the active layer 6 and thedislocation density originated in the active layer 6 increases withincreasing of lasing wavelength. At the lasing wavelength of 500 nm, thedislocation density in the p-side nitride semiconductor layer 8 is 2×10⁶cm⁻² or greater. From this, it can be seen that dislocations of 1.5×10⁶cm⁻² or greater is generated at the lasing wavelength of 500 nm.

Increase in the dislocation density in a laser diode element has beenknown to cause a decrease of the life characteristics, but the reasonfor the decrease of the life characteristics due to the increase of thedislocation density has been thought that the dislocations themselvesact as non-radiative recombination centers. Accordingly, the effort wasfocused on ways to reduce the generation of dislocations, in which athinner layers were employed to relax the distortions associated withlattice mismatch between the InGaN well layers 24 a, 24 b and theharrier layers 22 a, 22 b, or a smaller In content was studied to obtaina desired wavelength. However, for the aim to reduce the generation ofdislocations, if the thickness of the well layers 24 a, 24 b is reducedor other Group V element is added to the well layer, the bowingparameter is increased and which may lead to problems such as reductionin optical confinement in the well layers 24 a, 24 b, or deteriorationof crystal quality of the well layers 24 a, 24 b.

Meanwhile, according to a study conducted by the inventors ondeterioration behavior under high temperature operation (60° C.) of anitride semiconductor laser diode with a lasing wavelength of 506 nm, asshown in FIG. 6, a rapid deterioration took place within an initialseveral hours, which was followed by a large rate of deterioration.Examining the I-V characteristic of the nitride semiconductor laserdiode found that while a normal rectification characteristics is shownat the initial stage as shown in FIG. 7, an abnormal spike 52 isobserved at the rising portion of the current after a long period ofoperation, and the abnormality further expands during its operatinglife.

Based on the findings, the present inventor has carried out furtherinvestigations and discovered that, in a nitride semiconductor laserdiode having a lasing wavelength of 500 nm or greater, lifecharacteristics can be exponentially improved by controlling theconcentration distribution of p-type impurity in a depth direction.

FIG. 8 shows the depth profile of p-type impurity (Mg) concentration inthe nitride semiconductor laser diode whose life characteristics isshown in FIG. 6, measured by secondary ion mass spectrometer(hereinafter simply referred as “SIMS”). The left end shown in FIG. 8corresponds to the top surface of the p-side nitride semiconductor layer8 and the location corresponding to the top end of the second well layer24 b is shown by an arrow at approximately the center of FIG. 8. Thedistance 300 nm from the top end of the second well layer 24 b is shownby a dashed line. Within this range of distance, the holes can beefficiently supplied to the well layers 24 b and 24 a. The concentrationdistribution of the p-type impurity in this 300 nm range shows, from thewell layer 24 b toward the surface of the p-side nitride semiconductorlayer 8, that the concentration of the p-type impurity startedincreasing once entered in the p-side nitride semiconductor layer 2 fromthe active layer 6, and reaches a local maximum at a positioncorresponding to a location in the Al-containing nitride semiconductorlayer 26 (first p-type nitride semiconductor layer). The concentrationof the p-type impurity decreases beyond the local maximum 54 andexhibits a local minimum 56 at a position corresponding to a location ofthe p-side optical guide layers 28 a, 28 b (second p-type nitridesemiconductor layer).

Increasing the local maximum 54 and the local minimum 56 of the p-typeimpurity concentration enables an exponential improvement in the lifecharacteristics of the nitride semiconductor laser diodes. In theexample shown in FIG. 9, while employing a same structure as in thenitride semiconductor laser diode with the depth profile of the p-typeimpurity concentration shown in FIG. 6, the p-type impurityconcentration at the local maximum is set to 5×10¹⁸ cm⁻³ or greater andthe p-type impurity concentration at the local maximum is set to 6×10¹⁷cm⁻³ or greater. FIG. 10 shows the result of a high temperatureoperating life test at 60° C. of a nitride semiconductor laser diodewith the depth profile of the p-type impurity concentration shown inFIG. 9. A change in the driving current is not observed in 100 hours,and the estimated operating life under high-temperature was about 50,000hours. An estimated operating life is determined in such a way that therate of change of the driving current is measured at predetermineddriving conditions and based on the rate of change, and the duration oftime where the driving current is estimated to reach 1.3 times of theinitial stage.

FIG. 11A and FIG. 11B show the relationship between the p-type impurityconcentration and the estimated operating life at high temperatureoperation, at the local maximum 54 and local minimum 56 respectively. Asshown in FIG. 11A and FIG. 11B, with the p-type impurity concentrationat the local maximum of 5×10¹⁸ cm⁻³ or greater and the p-type impurityconcentration at the local minimum of 6×10¹⁷ cm⁻³ or greater, anexponential improvement in the life characteristics of the nitridesemiconductor laser diodes can be achieved.

The reason for the exponential improvement in the life characteristicsof the nitride semiconductor laser diodes by controlling the p-typeimpurity concentration is thought as below. When the In content in thewell layers 24 a, 24 b is increased to obtain thee lasing wavelength of500 nm or greater as described above, the dislocation density in thenitride semiconductor layer locates over the well layers 24 a, 24 bincreases to at least 1×10⁶ cm⁻². The crystal quality of the nitridesemiconductor layer in the vicinity of the dislocations deteriorates, sothat a partial inhibition to the generation of holes by the p-typeimpurity occurs with an increase of dislocation density. As a result,with a normal level of p-type impurity concentration, insufficientsupply of holes to the InGaN well layers 24 a, 24 b occurs. For thisreason, electrons diffuse into the p-side of the InGaN well layers 24 a,24 b and nonradiative recombination occurs. Also, the In content in theInGaN well layers 24 a, 24 b increases, which results in an increase ofdislocations from the InGaN well layers 24 a, 24 b. The dislocationsaffect the crystal quality of the p-side nitride semiconductor layer,and as a result, the life characteristics sharply deteriorate. However,maintaining the concentration of the p-type impurity greater than apredetermined value within the distance capable of efficiently supplyingcarriers to the well layers 24 a, 24 b (within 300 nm from the top ofthe well layer 24 b) enables to secure the supply of holes to the InGaNwell layers 24 a, 24 b. Light from the light emitting layer is stronglydistributed in a region about 300 nm from the top of the light emittinglayer. Therefore, once increase the concentration of the p-type impurityto greater than 5×10¹⁸ cm⁻³ and decrease the concentration of the p-typeimpurity to a range not less than 6×10¹⁷ cm⁻³ enables suppression of theabsorption of the emission by the p-type impurity and also tosuppression of increase of the threshold current. Accordingly, the lifecharacteristics of the nitride semiconductor laser diode can beexponentially improved.

In order to obtain a practical nitride semiconductor laser diode, theestimated operating life at 60° C. 5 mW of 5000 hours or greater,preferably 10000 hours or greater is desirable.

In the present embodiment, the concentration of the p-type impurity in arange 300 nm from the top of the well layer 24 b which is closer to thep-side nitride semiconductor layer 8 is controlled as described below:Dispose (a) a first p-type nitride semiconductor layer 26 made of anitride semiconductor containing Al having a larger band gap than thewell layers 24 a, 24 b with a concentration of the p-type impurity of5×10¹⁸ cm⁻³ or greater and (b) second p-type nitride semiconductorlayers 28 a, 28 b with a concentration of the p-type dopant within arange 300 nm from the top of the well layer 26 b less than that in thefirst p-type nitride semiconductor layer 26 and greater than 6×10¹⁷cm⁻³, in a range 300 nm from the top of the well layer 246 which isclosest to the p-side nitride semiconductor layer 8. The site of highestconcentration of the p-type impurity in the first p-type nitridesemiconductor layer 26 is the local maximum of the p-type impurity inthe concentration distribution in the depth direction. The site of thelowest concentration of the p-type impurity in the second p-type nitridesemiconductor layers 28 a, 28 b is the local maximum of the p-typeimpurity in the concentration distribution in the depth direction. Inthe case where a nitride semiconductor layer in which the concentrationof the p-type impurity is not distributed or the p-type impurity isadded at a smaller concentration than 6×10¹⁷ cm⁻³ is inserted within arange of 300 nm from the top of the well layer 24 b which is closest tothe p-side nitride semiconductor layer 8, there may be no problem if thethickness of the nitride semiconductor layer is sufficiently small. Forexample, even if an undoped nitride semiconductor layer is insertedwithin 300 nm from the top of the well layer 24 b, if the concentrationof the p-type impurity measured by SIMS to be described later is notless than 6×10¹⁷ cm⁻³ within a range of 300 nm from the top of the welllayer 24 b, requirements of the present invention will be satisfied.Because it is considered that the thickness of the undoped layer issmall to such degree where the concentration of the p-type impurity isnot less than 6×10¹⁷ cm⁻³ under a SIMS measurement with a conditionsexemplified in the present specification, with a diffusion of the p-typeimpurity from the layer adjacent to the undoped layer, the concentrationof the p-type impurity in the undoped layer can be of sufficient toobtain the effect of the present invention.

The concentration of p-type impurity in each layer can be controlled bythe flow rate of the raw material gas, growth temperature, pressure,V-III ratio, or the like of the p-type impurity in respectivevapor-phase growth. For example, the higher the flow rate of the rawmaterial gas of the p-type impurity and the growth temperature, thehigher the concentration of the p-type impurity becomes. In the casewhere the nitride semiconductor contains In, segregation of Inincreasingly occur with higher growth temperature. Therefore, an excessrise of the growth temperature is unfavorable. Also, there is a tendencythat, once starting to supply a raw material gas of p-type impurity at aconstant rate during vapor growth of the nitride semiconductor, theconcentration of the p-type impurity changes not in step-wise manner butincreases gradually with the growth of the nitride semiconductor. Also,even when a p-type impurity is doped in a layer at a constantconcentration, an influence is exerted by the concentration of p-typeimpurity in the adjacent layer. For example, if the concentration ofp-type impurity in the adjacent layer is high, a concentration gradientwith higher concentration of p-type impurity in the vicinity of theadjacent layer will occur by diffusion of the p-type impurity from theadjacent layer.

(Local Maximum of Concentration of p-type Impurity: First p-type NitrideSemiconductor Layer 26)

The local maximum value of the concentration of p-type impurity in thefirst p-type nitride semiconductor layer 26 is sufficient to be 5×10¹⁸cm⁻³ or greater, more preferably to be 7×10¹⁸ cm⁻³ or greater, andfurther preferably 1×10¹⁹ cm⁻³ or greater. The higher the p-typeimpurity concentration, the easier the holes to be injected in the welllayers 24 a, 24 b. On the other hand, if the concentration of p-typeimpurity is too high, the crystal quality of the first p-type nitridesemiconductor layer 26 deteriorates and absorption of light increases,which may result in an increase of threshold current. Therefore, thelocal maximum value of the concentration of p-type impurity in the firstnitride semiconductor layer 26 is preferably to be 1×10²⁰ cm⁻³ or less,more preferably to be 5×10¹⁹ cm⁻³ or less. The concentration of p-typeimpurity in the first p-type nitride semiconductor layer 26 may beconstant in a depth direction or may have distribution in the firstp-type nitride semiconductor layer 26. However, in the first p-typenitride semiconductor layer 26, the thickness of the region having theconcentration of p-type impurity of 5×10¹⁸ cm⁻³ or greater is preferably1 nm or greater, more preferably 5 nm or greater, and 20 nm or less, andmore preferably 50 nm or less. This is because if the region is toothick, deterioration of crystal quality or absorption of light becomes aproblem, and if the region is too thin, injection of holes in the welllayers 24 a, 24 b becomes insufficient.

In the present embodiment, the first p-type nitride semiconductor layer26 is made of a nitride semiconductor containing Al which has a largerband-gap than that of the well layers 24 a, 24 b. More preferably, it ismade of a nitride semiconductor layer having larger hand-gap than thatof the barrier layer 22 c. With this arrangement, the first p-typenitride semiconductor layer 26 functions as a carrier confinement layerconfining the electrons in the active layer, thus, preferable. It canalso function as a cap layer for preventing breakdown of the InGaNcrystal in the active layer 6. The first p-type nitride semiconductorlayer 26 may be a part of the p-side nitride semiconductor layer 8, ormay be a part of the active layer 6.

The local maximum of the concentration of the p-type impurity in thefirst p-type nitride semiconductor layer 26 to be preferably within adistance of 300 nm from the top of the well layer 24 b which is closerto the p-side nitride semiconductor layer 8, more preferably 150 nm orless and further preferably within 100 nm from the top of the well layer24 b. It is also preferable that the entire first p-type nitridesemiconductor layer 26 is in a predetermined distance. With thisarrangement, injection of the holes in the well layers 24 a, 24 b can befurther facilitated. The distance from the top of the well layer 24 bcan be adjusted by the thickness of the final barrier layer 22 c in thepresent embodiment. A layer other than the barrier layer 22 c may hedisposed between the well layer 24 b and the first p-type nitridesemiconductor layer 26. The local maximum of the concentration of p-typeimpurity may he formed not in the first p-type nitride semiconductorlayer 26 but in the barrier layer 22 c or the like, located beneath it.

(Local Minimum in p-type Impurity Concentration: Second p-side NitrideSemiconductor Layers 28 a, 28 b)

On the other hand, the local minimum value of 6×10¹⁷ cm⁻³ or greater inthe p-type impurity concentration in the second p-type nitridesemiconductor layers 28 a, 28 b is sufficient, but 8×10¹⁷ cm⁻³ orgreater is more preferable, and 1×10¹⁸ cm⁻³ or greater is furtherpreferable. The higher the p-type impurity concentration, the easier theholes to be injected in the well layers 24 a, 24 b. On the other hand,emission is relatively strongly distributed in a region about 300 nmfrom the top of the well layer 24 b which is close to the p-side nitridesemiconductor layer 8, so that if the concentration of p-type impurityin the second p-type nitride semiconductor layers 28 a, 28 b is toohigh, it may cause an increase of the threshold current. Particularly,in the present embodiment, the second p-type nitride semiconductorlayers 28 a, 28 b function as an optical guide layer of a separateconfinement heterostructure (SCH), so that effect of absorption of lightin the second p-type nitride semiconductor layers 28 a, 28 b becomesparticularly large. Thus, the local minimum value of the concentrationof p-type impurity in the second p-type nitride semiconductor layers 28a, 28 b is preferably ⅕ or less, more preferably 1/10 or less withrespect to the local maximum value of the concentration of p-typeimpurity in the first p-type nitride semiconductor layer 26. Also, thelocal minimum value of the concentration of p-type impurity in thesecond p-type nitride semiconductor layers 28 a, 28 b is preferably notgreater than 1×10¹⁹ cm⁻³, more preferably not greater than 5×10¹⁸ cm⁻³.

The concentration distribution of p-type impurity in the second p-typenitride semiconductor layers 28 a, 28 b is preferably such that afterpassing the local minimum described above, increases to 1×10¹⁸ cm⁻³ orgreater in a range of 300 nm from the top of the well layer 22 b.Absorption of light by the p-type impurity increases as the distancefrom the well layer 22 b decreases. Accordingly, in a range of 300 nmfrom the top of the well layer 22 b, arranging the concentration ofp-type impurity to decrease in a portion near the top of the well layer24 b so as to suppress absorption of light and to increase in a portionaway from the top of the well layer 24 b so as to increase theconcentration of p-type impurity of 1×10¹⁸ cm⁻³ or greater, injection ofthe holes in the well layers 24 a, 24 b can be further facilitated.

Such a distribution of p-type impurity can be realized by changing theflow rate of the row material gas of the p-type impurity during thevapor growth of the second p-type nitride semiconductor layers 28 a, 28b. For example, it may be such that dividing the second p-type nitridesemiconductor layers 28 a, b in the first layer 28 a and the secondlayer 28 b, and supplying the raw material gas of the p-type impuritywith a low flow rate or zero while growing the first layer 28 a, and ahigh flow rate while growing the second layer 28 b. Even if the flowrate of the raw material gas of the p-type impurity while growing thefirst layer 28 a is zero, the raw material gas of the p-type impuritysupplied during the growth of the first p-type nitride semiconductorlayer 26 beneath it remains. Therefore, the concentration of p-typeimpurity in the first layer 28 a gradually decreases as the distancefrom the interface with the first p-type nitride semiconductor layer 26increases, and becomes the local minimum at the interface with thesecond layer 28 b. Next, growing the second layer 28 b with supplyingthe raw material gas of p-type impurity, the concentration of p-typeimpurity can be increased.

In the present embodiment, the second p-type nitride semiconductorlayers 28 a, b is preferably made of a nitride semiconductor which has aband-gap smaller than that of the first p-type nitride semiconductorlayer 26 and the p-side cladding layer 32 to be grown next, and largerthan the well layers 24 a, h. For example, it can be formed with anitride semiconductor containing Al less than that in the first p-typenitride semiconductor layer 26 and the p-side cladding layer 32. Withthis arrangement, the second p-type nitride semiconductor layers 28 a, hcan serve as the p-side optical guide layer. The second p-type nitridesemiconductor layers 28 a, b may have other functions such as a claddinglayer in an element in which a optical guide layer is omitted.

The local minimum of the concentration of the p-type impurity in thesecond p-type nitride semiconductor layers 28 a, h to be preferablywithin a distance of 300 nm from the top of the well layer 24 b which iscloser to the p-side nitride semiconductor layer 8, more preferably 250nm or less and further preferably within 100 nm from the top of the welllayer 24 b. With this arrangement, injection of the holes in the welllayers 24 a, b can be further facilitated. The distance from the top ofthe well layer 24 b can be adjusted by the thickness of the finalbarrier layer 22 c, first p-type nitride semiconductor layer 26, and thefirst layer 28 a of the second p-type nitride semiconductor layer, inthe present embodiment. Also, a layer other than those layers describedabove may be interposed to adjust the distance. Also, the local minimumof the concentration of p-type impurity may be formed not in the secondp-type nitride semiconductor layers 28 a, h but in the first p-typenitride semiconductor layer 26 beneath those.

The longer the lasing wavelength of the nitride semiconductor laserdiode, the higher the density of dislocations generated in the activelayer 6. For this reason, the local maximum value of the concentrationof p-type impurity in the first nitride semiconductor 26 and the localminimum value in the second p-type nitride semiconductor layers 28 a, bare preferably set to a higher values as the lasing wavelengthincreases. For example, in the case where the lasing wavelength is 560nm, it is preferable to set the concentration of the p-type impuritywith the local maximum value of about 5×10¹⁹ cm⁻³ and the local minimumvalue of about 5×10¹⁸ cm⁻³. If the concentration of p-type impurityexceed those values, the crystal quality of the p-side nitridesemiconductor layer deteriorates and effect of absorption of emissionincreases. Accordingly, in the nitride semiconductor laser diodeaccording to the present invention, the lasing wavelength is desirablyset to 560 nm or less.

In the present invention, the depth distribution of the concentration ofp-type impurity can be measured by SIMS (ATOMIKA SIMS 4500). Forexample, with the primary ion species of O2+, the accelerating voltageof 2 kV, the electric current of 110 nA, and the raster region (etchedregion of the sample) of 120 μm², the primary ion is irradiatedperpendicularly to the sample, and the measuring region (obtaining thedata) is set to 30 μm² to detect the secondary ion, thus, themeasurement can be carried out. In order to determine the value of theconcentration of the p-type impurity based on a SIMS measurement, astandard sample of a nitride semiconductor layer of a known p-typeimpurity concentration is measured by SIMS and use the detected amountof the secondary ion as a standard. A standard sample can be made by,for example, ion-injecting a p-type impurity in a nitride semiconductorlayer. The dislocation density in the p-side nitride semiconductor layercan be measured by observing the upper surface of the p-side nitridesemiconductor layer with cathode luminescence (CL) or by observing across-section under a transmission electron microscope (TEM).

Now, each component of the nitride semiconductor laser diode 1 of thepresent embodiment will be described in detail below.

(Substrate 2)

The substrate 2 is preferably made of a nitride semiconductor greaterpreferably made of GaN. A substrate made of a nitride semiconductor hashigher heat conductivity than that of sapphire, so that heat dissipationefficiency can be improved and defects such as dislocations can bereduced, thus good crystal quality can be obtained. With a lowerdislocation density of the substrate 2, the state of surface of the welllayers 24 a, 24 b can be improved and the life characteristics can beimproved. Although a nitride semiconductor laser diode employing anInGaN light emitting layer exhibits a more gradual decline in lifecharacteristics caused by dislocations compared to othermaterial-systems, it still has dependency on dislocations. With a lowerdislocation density of the substrate 2, a higher ESD tolerance can beachieved. The dislocation density of the substrate 2 is preferably 1×10⁷cm⁻² or less, more preferably 5×10⁶ cm⁻² or less, and further preferably5×10⁵ cm⁻² or less. The dislocation density of the substrate 2 isconsidered as the dislocation density on the principal surface on whichthe nitride semiconductor layer to be grown.

Also, the dislocation density of the substrate 2 is preferably the sameor less than the dislocation density occurring in the active layer 6.When the dislocation density of the substrate 2 is the same or less thanthe dislocation density occurring in the active layer 6, thedistribution of dislocations in the entire element takes acharacteristic shape. That is, it takes a structure having a lowdislocation density from the substrate to the active layer and a highdislocation density above the active layer. In such a structure,introduction of dislocations enables release of distortion loaded fromthe under-layer to the active layer (well layer), so that reduction inthe light emitting efficiency caused by the distortion (for example,piezoelectric polarization etc.) can be suppressed, and is thuspreferable.

The substrate 2 made by various methods can be used. For example, thesubstrate may be obtained such a manner that, after growing a thicklayer of nitride semiconductor layer on a foreign substrate such assapphire by using hidride vapor phase epitaxy method (HVPF, method) andthen removing the foreign substrate to obtain the substrate made ofnitride semiconductor. Also, at the time of growing a nitridesemiconductor layer on a foreign substrate such as sapphire, dislocationdensity may be reduced by using a known lateral growth method. A wafercut out from an ingot of a nitride semiconductor crystal grown by usingan appropriate seed crystal may be used as the substrate 2.

Also, the laser diode is preferably grown on the C-plane of thesubstrate made of nitride semiconductor. Growing the laser diode on theC-plane of nitride semiconductor has advantages such that the cleavageplane (m-plane) is easily developed, the C-plane is chemically stablewhich facilitates processing and has resistance to etching to somedegree sufficient for processing in the laser steps.

(n-side Nitride Semiconductor Layer)

The layer structure of the n-side nitride semiconductor layer 4 used inthe present embodiment is shown in FIG. 12. Of the layers describedbelow, the layers other than the n-side cladding layer 16 can be omittedaccording to the structure of the element.

First, a first n-side nitride semiconductor layer 12 made of a nitridesemiconductor having a thermal expansion coefficient smaller than thatof the nitride semiconductor constituting the substrate and doped withan n-type impurity such as Si is grown on the substrate 2 made of anitride semiconductor. The first n-side nitride semiconductor layer 12serves as the under-layer. The first n-side nitride semiconductor layer12 is made of a nitride semiconductor containing Al, preferably made ofAlGaN. Forming the first n-side nitride semiconductor layer 12 with amaterial having a thermal expansion coefficient smaller than that of thenitride semiconductor constituting the substrate enables application ofcompression strain on the first n-side nitride semiconductor layer 12which enables prevention of generation of microscopic crackes. The firstn-side nitride semiconductor layer 12 is preferably made with athickness of 0.5 to 5 μm.

On the first n-side nitride semiconductor layer 12, a second n-sidenitride semiconductor layer 14, made of a nitride semiconductorcontaining In, preferably made of InGaN, and doped with an n-typeimpurity is grown. The second n-side nitride semiconductor layer 14 canbe served as a crack prevention layer. A nitride semiconductorcontaining In has relatively flexible crystal, which enables relaxing ofthe distortion applied on the nitride semiconductor layer grown on it,so that occurrence of cracks can be prevented. The second n-side nitridesemiconductor layer 14 is preferably formed with a thickness of 50 to200 nm.

On the second n-side nitride semiconductor layer 14, a third n-sidenitride semiconductor layer 16 made of a nitride semiconductorcontaining Al, preferably made of AlGaN, and doped with a n-typeimpurity is grown. The third n-side nitride semiconductor layer 16 canserve as the n-side cladding layer. The third n-side nitridesemiconductor layer 16 is constituted with a nitride semiconductorhaving a band gap at least larger than that of the barrier layers 22 a,22 b, 22 c. The third n-side nitride semiconductor layer 16 may eitherbe a single layer or a multilayer. Also, it may be a multilayer having asuperlattice structure. The third n-side nitride semiconductor layer 16is preferably made with a thickness of 0.5 to 2.0 μm.

On the third n-side nitride semiconductor layer 16, fourth n-sidenitride semiconductor layers 18 a, 18 b each having a band-gap smallerthan that of the third n-side nitride semiconductor Layer 16 and largerthan that of the well layers 24 a, 24 b are formed. The fourth n-typenitride semiconductor layers 18 a, 18 b can serve as the n-side opticalguide layer. The fourth n-side nitride semiconductor layers 18 a, 18 bare preferably made of GaN or InGaN. The fourth n-side nitridesemiconductor layers 18 a, 18 b sufficiently supply electrons to theactive layer 6 while suppressing absorption of light. Therefore, it ispreferable to divide into (i) a fourth n-side nitride semiconductorlayer 18 a away from the active layer and grown by being doped with ann-type impurity and (ii) a fourth n-side nitride semiconductor layer 18b close to the active layer 6 and grown without being grown by an n-typeimpurity. The fourth n-side nitride semiconductor layers 18 a, 18 b arepreferably formed with a total thickness of 200 to 300 nm.

(Active Layer 6)

The active layer 6 is sufficient to have a light emitting layerincluding In_(x)Al_(y)Ga_(1−x−y)N (0<x<1, 0≦y<1, 0<x+y<1) and the activelayer having the multiquantum well structure shown in FIG. 1 or other,such as an active layer having a single quantum well structure, anactive layer made of a thin single light emitting layer, or the like canbe used. In the quantum well structure, the well layers 24 a, 24 b serveas the light emitting layer. The light emitting layer is sufficient tocontain In_(x)Al_(y)Ga_(1−x−y)N (0<x<1, 0≦y<1, 0<x+y<1), but morepreferably made of InGaN. In the present invention, the term “lightemitting layer” refers to a layer in which radiative recombination ofholes and electrons occurs.

The emission wavelength of the light emitting layer can be adjusted bythe In content, as described more specifically in examples. When thewell layer has a high In content, a well-cap layer (not shown) ispreferably provided over each well layer to prevent the deterioration ofthe well layer. It is preferable that the well-cap layer has a thicknessin a range of 1-5 nm, and made of AlInGaN with the Al content of 0-50%,more preferably made of AlGaN with the Al content of 0-30%. The well-caplayer is formed between the well layer and the barrier layer.

The light emitting layer of the active layer 6 with a smaller thicknessis capable of reducing the threshold current and facilitating thereleasing of the lattice constant mismatching with the barrier layer,but with an excessively small thickness fails to provide a sufficientcarrier confinement. For this reason, the thickness of the lightemitting layer is preferably 1.0 nm or greater, more preferably 2.0 nmor greater and preferably 5.0 nm or less, more preferably 4.0 nm orless. The light emitting layer of the active layer 6 may be doped withor not doped with an n-type impurity. However, the crystal quality of anitride semiconductor containing In tends to deteriorate with a increaseof the n-type impurity concentration, so that the n-type impurityconcentration is preferably kept low to obtain the light emitting layerwith good crystal quality.

Forming the active layer with a multiquantum well structure enablesimprovement of the output power, reduction of the lasing thresholdvalue, or the like. When the active layer 6 is made of a multiquantumwell structure, the first and the last layers may either be a well layeror a barrier layer as long as well layers and barrier layers arealternately stacked. However, as in the present embodiment, theoutermost layer is preferably a barrier layer. That is, in amultiquantum well structure, the barrier layer interposed in the welllayers is not specifically limited to be a single layer (welllayer/barrier layer/well layer) and two or greater barrier layers, suchas in “well layer/barrier layer (1)/barrier layer (2)/ . . . /welllayer,” a plurality of harrier layers with different compositions andamounts of impurity may be provided.

The barrier layers 22 a, 22 b, 22 c used in the active layer 6 having aquantum well structure are not specifically limited, but a nitridesemiconductor having a lower In content than in the well layers 24 a, 24b, or GaN, a nitride semiconductor containing Al, or the like, can beused. It is more preferable to include InGaN, GaN or AlGaN. Thethickness and composition of the barrier layers 22 a, 22 b, and 22 c arenot necessarily to be the same in the quantum well structure. Thus, inthe present embodiment, the thickness is increased in the order of thebarrier layer 22 a closest to the n-side>the barrier layer 22 c closestto the p-side>the harrier layer 22 b interposed therebetween. Also, then-type impurity is only doped in the barrier layer 22 a which is closestto the n-side, and other barrier layers 22 b, 22 c and the well layers24 a, 24 b are grown without doped with an n-type impurity.

In the present embodiment, the number of the well layers 24 a, 24 b isset to two and the number of the barrier layers 22 a, 22 b, 22 c is setto three, but the present invention is not limited to those. Forexample, the number of the well layers 24 a, 24 b may not be two but maybe increased to such as three layers or four layers. Generally, thelonger the lasing wavelength, the smaller the thickness of the welllayer needed to be to prevent occurrence of dislocations in the activelayer 6. In the present invention, generation of dislocations in theactive layer 6 is allowed. Therefore, there is no need to reduce thethickness of the well layer to the degree that results in insufficientcarrier confinement as the entire active layer 6. However, withincreasing the number of the well layers, carrier confinement as theentire active layer 6 can be achieved even with using the well layerswith much smaller thickness. Accordingly, excess generation ofdislocations in the active layer 6 can be avoided. In the case of anitride semiconductor laser diode with the lasing wavelength of 500 nmor greater, the threshold current decreases with the number of the welllayers three layers or four layers, compared to the case with twolayers.

The lasing wavelength of the active layer 6 according to the presentinvention is sufficient to be 500 nm or greater, but an excessively longwavelength is not preferable for it results in excessively highdislocation density generated in the active layer 6. As shown in FIG. 5,when the lasing wavelength of the active layer 6 is 560 nm, the densityof dislocations generated in the active layer 6 may be about 1×10⁷ cm⁻².The dislocation density of more than the above is not preferable becausethe necessary amount of the concentration of the p-type impurity becomestoo high. For this reason, the lasing wavelength of the active layer 6is set to 560 nm or less so that the dislocation density generated inthe active layer 6 to be 1×10⁷ cm⁻².

(p-side Nitride Semiconductor Layer 8)

As the p-side nitride semiconductor layer 8, an Al-containing nitridesemiconductor layer 26 (first p-type nitride semiconductor layer), ap-side optical guide layer 28 a, 28 b (second p-type nitridesemiconductor layer), a p-side cladding layer 32 (third p-type nitridesemiconductor layer), and a p-side contact layer 34 (fourth p-typenitride semiconductor layer) are stacked. The layers other than thep-side cladding layer 32 can be omitted according to the element. Thep-side nitride semiconductor layer 8 is needed to have a band-gap widerthan that of the active layer 6 at least at a portion being in contactwith the active layer 6, and for this reason, a composition containingAl is preferable. Each layer may be grown while being doped with ap-type impurity to obtain the p-type, or a p-type impurity is diffusedfrom the adjacent other layers to obtain the p-type. For the p-typeimpurity, Be, Zn, Cd, or the like can be used as other than Mg.

The Al-containing nitride semiconductor layer 26 is made of a p-typenitride semiconductor having an Al mixed crystal ratio higher than thatof the p-side cladding layer 32, and preferably contains Al_(x)Ga_(1−x)N(0.1<x<0.5). Also, a p-type impurity such as Mg is doped at aconcentration of 5×10¹⁸ cm⁻³ or greater. With this arrangement, theAl-containing nitride semiconductor layer 26 is able to sufficientlyconfine electrons in the active layer 6, so that the threshold value ofthe laser can be reduced. Also, the Al-containing nitride semiconductorlayer 26 is sufficiently grown as a thin film with a thickness of about3 to 50 nm, more preferably about 3 to 20 nm. Such a thin film can begrown at a lower temperature than that of the p-side optical guide layer28 a, 28 b and the p-side cladding layer 32. Therefore, with forming theAl-containing nitride semiconductor layer 26, deterioration of theactive layer 6 containing In can be suppressed compared to the casewhere the p-side optical guide layers 28 a, 28 b and the like aredirectly disposed on the active layer 6.

The Al-containing nitride semiconductor layer 26 also diffusionsupplying p-type impurity to the barrier layer 22 c which is grownundoped. They both work together to protect the well layers 24 a, 24 bfrom deterioration and to enhance efficiency in injecting holes in thewell layers 24 a, 24 b. That is, as the last layer of the active layer6, an undoped barrier layer 22 c is disposed with a thickness largerthan that of the other barrier layers, and thereon, a nitridesemiconductor layer 26 having a small thickness and containing anundoped p-type Al_(x)Ga_(1−x)N (0.1<x<0.5) is grown at a lowtemperature. Accordingly, the active layer 6 containing In can beprotected from deterioration and the p-type impurity such as Mg can bediffused from the p-type Al_(x)Ga_(1−x)N layer to the undoped barrierlayer 22 c, so that the p-type impurity such as Mg diffuse from the p-type N layer to the undoped barrier layer injection efficiency of holesin the active layer 6 can be improved.

The Al-containing nitride semiconductor layer 26 is to serve as anelectron confinement layer, so that is disposed between the active layer6 and the cladding layer 32. In the case where the optical guide layers28 a, 28 b are further provided, the Al-containing nitride semiconductorlayer 26 is preferably disposed between the optical guide layers 28 a,28 b and the active layer 6. Forming the Al-containing nitridesemiconductor layer 26 within 300 nm, more preferably within 200 nm, andfurther preferably within 100 nm from the top of the well layer 24 bwhich is closest to the p-side nitride semiconductor layer 8 allows theAl-containing nitride semiconductor layer 26 to serve as an electronconfinement layer and enables efficient supply of the holes. The closerthe Al-containing nitride semiconductor layer 26 to the active layer 6,the more efficient the effect of carrier confinement. Moreover, in mostcases, the laser element another layer is not specifically neededbetween the Al-containing nitride semiconductor layer 26 and the activelayer 6, so that generally it is most preferable to provide anAl-containing nitride semiconductor layer 26 in contact with the activelayer 6. Also, it is possible to provide a buffer layer therebetween.

The p-side light guiding layers 28 a, 28 b (second p-type nitridesemiconductor layer) are preferably made of a p-type nitridesemiconductor layer 26 containing Al and a nitride semiconductor whichhas a band-gap smaller than that of the p-type cladding layer 32 to begrown next, and larger than that of the well layers 24 a, 24 b. Forexample, it can be formed with a nitride semiconductor containing Alless than that in the p-type nitride semiconductor layer 26 containingAl and the p-side cladding layer 32. In order to sufficiently supplyholes to the active layer 6 while preventing absorption of light, thep-side optical guide layer 28 a, 28 b are preferably divided into (i) afirst p-side optical guide layer 28 a disposed closer to the activelayer 6 and grown without being doped with a p-type impurity, and (ii) asecond optical guide layer 28 b disposed away from the active layer 6and grown while being doped with a p-type impurity.

The p-side cladding layer 32 (third p-type nitride semiconductor layer)preferably made with a superlattice structure including an Al-containingnitride semiconductor layer, preferably Al_(x)Ga_(1−x)N (0<x<1), furtherpreferably made with a superlattice structure in which AlGaN ofdifferent Al composition are stacked. Providing the p-side claddinglayer 32 with a superlattice structure allows increase of the Al mixedcrystal ratio of the entire cladding layer which enables decrease of therefractive index of the cladding layer, and further increase of the bandgap energy. Thus, it is significantly advantageous for decreasing thethreshold. Further, with a superlattice structure, pits and cracksoccurring in the cladding layer can be reduced compared to that withouta superlattice structure, so that occurrence of short circuit can alsobe reduced. The band gap of the p-side cladding layer 32 (third p-typenitride semiconductor layer) is preferably larger than that of thep-side optical guide layer 28 a, 28 b (second p-type nitridesemiconductor layer) and smaller than that of the first p-type nitridesemiconductor layer 26. The band gap of the p-side cladding layer 32(third p-type nitride semiconductor layer) having a superlatticestructure made by stacking an A layer and a B layer can be considered anaverage of the A layer and the B layer. The p-type impurityconcentration of the p-side cladding layer 32 (third p-type nitridesemiconductor layer) is preferably higher than that of the p-sideoptical guide layers 28 a, 28 b (second p-type nitride semiconductorlayer) and lower than that of the p-side contact layer 34 (fourth p-typenitride semiconductor layer). The higher p-type impurity concentrationthan that of the p-side optical guide layers 28 a, 28 b (second p-typenitride semiconductor layer) enables aid of the supply of holes and thelower p-type impurity concentration than that of the p-side contactlayer 34 (fourth p-type nitride semiconductor layer) enables suppressionof increasing of the threshold current caused by deterioration ofcrystal quality. The impurity concentration of the p-side cladding layer32 (third p-type nitride semiconductor layer) having a superlatticestructure made by stacking an A layer and a B layer can be considered anaverage of the A layer and the B layer.

The p-side contact layer 34 (fourth p-type nitride semiconductor layer)can be constituted with a p-type nitride semiconductor, and preferably,GaN doped with Mg is employed to obtain good ohmic contact with thep-side electrode 38. The p-side contact layer 34 is to form anelectrode, so that preferably formed with a high carrier concentrationof 5×10¹⁹/cm³ or greater.

In the laser diode according to the present embodiment, after forming aridge 36 by etching a part of the optical guide layers 28 a, b, the sidesurfaces of the ridge 36 are covered with an insulating embedded layersuch as SiO₂, and an insulating protective film 48 such as SiO₂ isfurther provided. For the protective film 48, a semi-insulating, i-typenitride semiconductor, a nitride semiconductor having an oppositeconductivity to that of the ridge portion, or the like, can also beused.

At the time of providing the ridge 36, as shown in FIG. 13, a groove 49in parallel to the ridge is preferably formed at the both sides of thebottom portion of the ridge 36. When the In content of the well layer ishigh, the difference in the refractive indices to AlGaN which istypically used as the p-side cladding layer 32 decreases, so that asufficient light confinement index is difficult to obtain. For thisreason, providing a groove 49 which is continuous in the lengthwisedirection of the resonator at the both sides of the ridge 36 enablessufficient optical confinement even in the case where the In content ofthe well layer is high.

The laser diode according to the present invention is not limited to arefractive index waveguide type having a ridge structure described aboveand various structures can be employed such as a BH structure in whichthe side surfaces of the ridge are embedded by re-growth or a structureprovided with a current confinement layer. The laser diode according tothe present invention is made of a nitride semiconductor represented byAl_(x)In_(y)Ga_(1−x−y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1), which is preferably anitride semiconductor of a hexagonal crystal system. With a hexagonalcrystal system, other elements may be contained in a Group III elementor a Group V element at a small amount which does not reduce the crystalquality.

According to the present invention, a nitride semiconductor laser diodewith a lasing wavelength of 500 nm and has excellent emission efficiencyand life characteristics can be realized. With the green nitridesemiconductor laser diode of the present invention, a full-color displaycan be realized by a combination of a conventional blue nitridesemiconductor laser diode and a conventional red semiconductor laserdiode. For example, combining the nitride semiconductor laser diodehaving a lasing wavelength of 500 to 560 nm constituted according to thepresent invention with a nitride semiconductor laser diode having alasing wavelength of 440 to 480 nm and a semiconductor laser diodehaving a lasing wavelength of 600 to 660 nm, a full-color displayapparatus using semiconductor lasers can be obtained.

EXAMPLES Example 1

A nitride semiconductor laser having a structure shown in FIG. 1 ismanufactured as described below.

(n-side Nitride Semiconductor Layer 4)

First a gallium nitride substrate 2, which has a C-plane as a principalsurface, with dislocation density of about 5×10⁵ cm⁻² is prepared. Then,an n-type Al_(0.03)Ga_(0.97)N layer 12 doped with Si at 3×10¹⁸ cm³ isgrown to a thickness of 2 μm (first n-side nitride semiconductor layer)by using TMG (trimethyl gallium, TMA (trimethyl aluminum), SiH4(silane), and ammonia at 1140° C. with hydrogen as carrier gas by usingMOCVD on the gallium nitride substrate 2. Following that, at thetemperature of 930° C. and using TMI (trimethyl indium), an n-typeIn_(0.06)Ga_(0.94)N layer 14 doped with Si at 4×10¹⁸ cm³ is grown to athickness of 0.15 μm (second n-side nitride semiconductor layer). Next,at the temperature of 990° C., an n-type Al_(0.09)Ga_(0.91)N layer 16doped with Si at 2×10¹⁸ cm³ is grown to a thickness of 1 μm (thirdn-side nitride semiconductor layer 16). This layer can also be made witha multilayer structure such as Al_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N (0≦x≦1,0≦y≦1) with an appropriate thickness ratio with an average Al-content of9%. Next, TMA is stopped and at the temperature of 990° C., an n-typeGaN layer 18 a doped with Si at 1×10¹⁸ cm³ and an undoped n-type GaNlayer 18 b are respectively grown to a thickness of 0.15 μm (fourthn-side nitride semiconductor layer 18 a, 18 b). The undoped GaN layer 18b may be doped with an n-type impurity.

(Active Layer 6)

Next, an active layer 6 is grown as follows. The carrier gas is changedto nitrogen and at the temperature of 925° C., a barrier layer 22 a ofIn_(0.04)Ga_(0.96)N doped with Si of 2×10¹⁸/cm³ and an undoped GaN layer(not shown) are grown to a thickness of 210 nm and 1 nm respectively.Next, at the temperature of 780° C., after growing a well layer 24 a ofundoped In_(0.23)Ga_(0.77)N to a thickness of 3 nm, a well-cap layer(not shown) of undoped GaN is grown to a thickness of 1 nm. Then, thetemperature is raised to 925° C., a barrier layer 22 b of undoped GaN isgrown to a thickness of 14 nm. Next, at the temperature of 780° C., awell layer 24 b of undoped In_(0.23)Ga_(0.77)N is again grown to athickness of 3 nm, and thereafter, a well-cap layer (not shown) ofundoped GaN is grown to a thickness of 1 nm. Then, the temperature israised to 925° C., a barrier layer 22 c of undoped In_(0.04)Ga_(0.96)Nis grown to a thickness of 70 nm. Thus, an active layer having amultiquantum well structure (MQW) is formed.

(p-side Nitride Semiconductor Layer 8)

Next, the temperature is raised to 990° C. and the carrier gas ischanged from nitrogen to hydrogen, and using Cp₂Mg (bis-cyclopentadienylmagnesium) as a Mg impurity, a p-type Al_(0.2)Ga_(0.8)N layer 26 (firstp-type nitride semiconductor layer) doped with Mg of 1×10¹⁹/cm³ is grownto a thickness of 10 nm. In the layer 26, the Al compositiondistribution in the growth direction is within 0 to 20%. Then, at 990°C. an undoped p-type Al_(0.03)Ga_(0.97)N layer 28 a and a p-typeAl_(0.003)Ga_(0.97)N layer 28 b doped with Mg of 3×10¹⁸/cm³ arerespectively grown to a thickness of 0.15 pm (second p-type nitridesemiconductor layer). In the undoped layer 28 a, Mg is not intentionallydoped. However, Mg used in the immediately preceding growth of the layer26 remains in the MOCVD reaction chamber and is incorporated in thelayer 28 a during the growth, and results in the Mg concentration in thelayer 28 a of 1.2×10¹⁸ cm³ or greater. The layer 28 a may beintentionally doped with Mg, or the Al composition thereof may be 0 to3%. Next, a 2.5 nm thickness of Mg-doped Al_(0.06)Ga_(0.94)N layer, a2.5 nm thickness of undoped Al_(0.12)Ga_(0.88)N layer are alternatelygrown, so that the third layer 32 (third p-type nitride semiconductorlayer) of a total thickness of 0.45 μm is grown. The average Mgconcentration in the layer 32 is about 1×10¹⁹ cm ³. Finally, at 990° C.,a p-type GaN layer 32 doped with Mg of 1×10²⁰ cm³ is grown to athickness of 15 nm on the layer 26.

Next, the wafer on which the nitride semiconductor has grown is takenout of the reaction vessel, and a mask made of SiO₂ is formed on thesurface of the uppermost layer of the p-type GaN layer 32. Then usingthe mask, 3 μm of etching is carried out on the nitride semiconductorlayer and a stripe structure with a length of 600 μm (corresponding tothe length of resonator) is formed. This portion serves as the resonatorcavity of the laser element. The length of the resonator is preferablyin a range of 200 μm to 5000 μm. Next, a mask with a stripe shape madeof SiO₂ is formed on the p-type GaN layer 32, and using the mask,etching is carrier out on the p-type GaN layer 32 by way of RIE(Reactive Ion Etching). With this, the ridge portion 36 which is astripe-shaped waveguide region is formed with a width of 2 μm. At thistime, the etching conditions (pressure, temperature) are adjusted sothat, as shown in FIG. 13, the side portions 49 of the ridge is etched30 nm deeper than the peripheral portions of the ridge and the sidewalls of the ridge portion 36 are formed at an angle of 75 degrees withrespect to the p-type GaN layer 32.

Next, the entire upper surface of the wafer is covered with aphotoresist and etching is carried out on the photoresist until the SiO₂on the ridge portion 36 is exposed. Then, a mask of a photoresist isagain applied on the regions corresponding to the end surfaces of theresonator when divided into chips, and etching is carried out on theSiO₂ on the ridge 36 except for the area around the end surfaces and thep-type GaN layer is exposed. Next, a p-electrode 38 made of Ni(10nm)/Au(100 nm)/Pt(100 nm) is formed over the entire wafer by way ofsputtering. Thereafter, the entire photoresist is removed so that thep-electrode 38 only remain at the portions of the stripe-shaped ridge 36where the SiO₂ has been removed. Then, ohmic annealing is carried out at600° C.

Next, an embedded layer 46 made of a Si oxide (SiO₂, 200 nm) is disposedby way of sputtering. At this time, the ridge portion 36 has a taperedshape and the cross-sectional area with respect to the surface of thewafer of the side walls of the ridge portion 36 is smaller than otherregions, so that the thickness of the embedded layer 46 is adjustedaccording to the relationship of the side wall portions of: the ridgeportion<the side portions 49 of the ridge. The density of the reactiveion is decreased as the formation of the film on the side walls of theridge progresses, and the growth rate of the film decreases more thanthat in the regions other than the ridge. Therefore, the embedded layer46 is formed with a thickness in the relationship of side portions 49 ofridge<regions other than ridge. Accordingly, the thickness of theembedded layer 46 has a relationship of: side wall portions ofridge<side portions 49 of ridge<regions other than ridge.

Next, the entire upper surface of the wafer is again covered with aphotoresist and etching is carried out on the photoresist until thep-electrode 38 on the ridge portion 36 is exposed. Then, the SiO₂ on thep-electrode 38 and the end surface portion of the resonator is removed.Next, a protective film 48 made of a Si oxide (SiO₂) is deposited bysputtering with a thickness of 0.5 μm, over the embedded film and on theside surfaces of the semiconductor layer.

Next, continuously to the p-electrode 38 exposed in the previous step, ap-pad electrode 40 made of Ni(8 nm)/Pd(200 nm)/Au(800 nm) is formed.Next, polishing is carried out from the opposite surface to the growthsurface of the nitride semiconductor layer so as to obtain a thicknessof the substrate 2 of 80 μm. Next, on the polished surface, ann-electrode 42 made of V(10 nm)/Pt(200 nm)/Au(300 nm) is formed. Next,the wafer is divided into a piece-wise shape by using laser and cleavedalong the (1-100) plane as the cleavage surface to form the resonatorend surfaces. Next, a mirror made of SiO₂/ZrO₂ is formed on the both ofthe end surfaces. Finally, the bar-shaped wafers are cut perpendicularto the resonator end surfaces to obtain semiconductor laser elements.

In the nitride semiconductor laser diode thus formed, dislocations ofabout 2×10⁶ cm⁻² occur from the active layer 6. The Mg concentrationprofile in the p-side nitride semiconductor layer 8 shows, as in Example1, Mg concentration of a local maximum value of 1×10¹⁰/cm³ and a localminimum value of 1.2×10¹⁸/cm³. The lasing wavelength is 500 nm, thethreshold current is 25 mA, and the output power is 5 mW. Life test at60° C., APC, and 5 mW shows a result as shown in FIG. 10, the estimatedoperating life at 60° C. is about 50,000 hours.

Comparative Example 1

The concentration of Mg incorporated in the p-type Al_(0.2)Ga_(0.8)Nlayer 26 (first p-type nitride semiconductor layer) is 4.3×10¹⁸ cm⁻³,the concentration of Mg incorporated in the p-type Al_(0.03)Ga_(0.97)Nlayers 28 a, 28 b (second p-type nitride semiconductor layer) is5.6×10¹⁷ cm⁻³, 1.4×10¹⁸cm⁻³ respectively. The nitride semiconductorlaser is manufactured in the same manner as in Example 1 except for thatdescribed above. Dislocations of about 2×10⁶ cm⁻² occur from the activelayer 6. The Mg concentration profile in the p-side nitridesemiconductor layer 8 is as shown in FIG. 8. the Mg concentration has alocal maximum value of 4.3×10¹⁸/cm⁻³ and a local minimum value of5.6×10¹⁷/cm⁻³. The lasing wavelength is 500 nm, the threshold current is30 mA, and output is 5 mW. Life test at 60° C. APC, and 5 mW shows aresult as shown in FIG. 6, the operating life at 60° C. is about 40hours.

Example 2

In Example 1, the composition of the well layer is In_(0.25)Ga_(0.73)N,with a thickness of 2.5 nm, and the number of the well layers is 3. Theconcentration of Mg incorporated in the p-type Al_(0.2)Ga_(0.8)N layer26 (first p-type nitride semiconductor layer) is 2×10¹⁹ cm⁻³, theconcentration of Mg incorporated in the p-type Al_(0.03)Ga_(0.97)Nlayers 28 a, 28 b (second p-type nitride semiconductor layer) is 2×10¹⁸cm⁻³, 4×10¹⁸ cm⁻³ respectively. The nitride semiconductor laser ismanufactured in the same manner as in Example 1 except for thatdescribed above. Dislocations of about 5×10⁶ cm⁻² occur from the activelayer 6. The lasing wavelength is 517 nm, the threshold current is 70mA, and output is 5 mW. Life test at 60° C., APC, and 5 mW shows anestimated operating life is 10,000 hours or greater.

Example 3

In Example 1, the composition of the well layer is In_(0.27)Ga_(0.73)N,with a thickness of 2.2 nm, and the number of the well layers is 4. Theconcentration of Mg incorporated in the p-type Al_(0.2)Ga_(0.8)N layer26 (first p-type nitride semiconductor layer) is 3×10¹⁹ cm⁻³, theconcentration of Mg incorporated in the p-type Al_(0.03)Ga_(0.97)Nlayers 28 a, 28 b (second p-type nitride semiconductor layer) is 3×10¹⁸cm⁻³, 6×10¹⁸ cm⁻³ respectively. The nitride semiconductor laser ismanufactured in the same manner as in Example 1 except for thatdescribed above. Dislocations of about 7.5×10⁶ cm⁻² occur from theactive layer 6. Lasing occurs at 540 nm, and a life test at 60° C., APC,and 5 mW shows a significantly preferable operating life characteristicscompared to that of Comparative Example.

Example 4

In Example 1, the composition of the well layer is In_(0.3)Ga_(0.7)N,with a thickness of 2 nm, and the number of the well layers is 4. Theconcentration of Mg incorporated in the p-type Al_(0.2)Ga_(0.8)N layer26 (first p-type nitride semiconductor layer) is 5×10¹⁹ cm⁻³, theconcentration of Mg incorporated in the p-type Al_(0.03)Ga_(0.97)Nlayers 28 a, 28 b (second p-type nitride semiconductor layer) is 5×10¹⁸cm⁻³, 1×10¹⁹ cm⁻³ respectively. The nitride semiconductor laser ismanufactured in the same manner as in Example 1 except for thatdescribed above. Dislocations of about 1×10⁷ cm⁻² occur from the activelayer 6. Lasing occurs at 560 nm, and a life test at 60° C., APC, and 5mW shows a significantly preferable operating life characteristicscompare to that of Comparative Example.

Example 5

In Example, growth of the p-type Al_(0.03)Ga_(0.97)N layers 28 a, 28 b(second p-type nitride semiconductor layers) is omitted and a layer 32having the thickness of 0.45 μm is grown directly on the p-typeAl_(0.2)Ga_(0.8)N layer 26 (first p-type nitride semiconductor layer).The first 0.15 μm of the layer 32 is grown with undoped layers ofAl_(0.06)Ga_(0.94)N layer and Al_(0.12)Ga_(0.88)N layer, and the rest ofthe 0.3 μm is grown with Al_(0.06)Ga_(0.94)N layer doped with Mg of aconcentration the same as in Example 1. Because the layer 32 has alarger band gap (i.e. smaller refractive index) than that of the layers28 a, 28 b, the optical confinement is enhanced and also the opticalabsorption due to doping with Mg can be reduced. Dislocations of about2×10⁶ cm⁻² occur from the active layer 6. The Mg concentration profilein the p-side nitride semiconductor layer 8 shows, as in Example 1, Mgconcentration of a local maximum value of 1×10¹⁹/cm³ and a local minimumvalue of 1.2×10¹⁸/cm³. Lasing occurs at 500 nm. Although the thresholdcurrent is rather high than that of Example 1, a life test at 60° C.,APC, and 5 mW shows nearly the same operating life as in Example 1.

Example 6

In Example 1, the Mg concentration in the p-type Al_(0.03)Ga_(0.07)Nlayers 28 a, 28 b (second p-type nitride semiconductor layer) is set toa constant value of 3×10¹⁸/cm⁻³as shown below. After the layer 26 isgrown, maintaining the growth temperature at a constant level, a Mg rawmaterial is supplied only for the beginning of growth of 10 nm, then thesupply of the Mg raw material is stopped. After about 50 nm of thegrowth from there, the supply of the Mg raw material is restarted, thenthe flow of the Mg raw material gas is gradually reduced. With this, thep-type Al_(0.03)Ga_(0.97)N layers 28 a, 28 b can be formed with the Mgconcentration constantly distributed in the entire layers. As in Example1, dislocations of about 2×10⁶ cm⁻² occur from the active layer 106. TheMg concentration profile in the p-side nitride semiconductor layer 8shows, Mg concentration of a local maximum value of 1×10¹⁹/cm³, butthereafter, shows a constant value of 3×10¹⁸/cm³ in the p-typeAl_(0.03)Ga_(0.97)N layers 28 a, 28 b. Easing occurs at 500 nm. Althoughthe threshold current reaches 50 mA, a life test at 60° C., APC, and 5mW shows nearly the same operating life as in Example 1.

Example 7

In Example 1, the barrier layer 22 c is grown so that the band gapincreases in a continuous manner or a stepwise manner. In the samemanner as in Example 1, the barrier layer 22 c made of undopedIn_(0.04)Ga_(0.96)N is grown to 70 nm, and then, an undoped AlInGaNlayer (for example GaN, AlGaN) having larger hand gap than the harrierlayer 22 c is grown to a thickness of 20 nm. Thereafter, in the samemanner as in Example 1 the p-side nitride semiconductor layer 8 isgrown. With this arrangement, optical confinement is enhanced without anincrease of the thickness of the p-side nitride semiconductor layer 8,so that operating voltage decreases. A life test at 60° C., AOC, and 5mW shows nearly the same operating life as in Example 1.

Example 8

In Example 1, the growth of layer 32 (third p-type nitride semiconductorlayer), in which a 2.5 nm thickness of Mg-doped Al_(0.06)Ga_(0.94)Nlayer, a 2.5 nm thickness of undoped Al_(0.12)Ga_(0.88)N layer arealternately grown, is omitted. Also, ITO is used as the p-electrode inplace of Ni/Au/Pt. The nitride semiconductor laser is manufactured inthe same manner as in Example 1 except for that described above. Thep-electrode 38 made of ITO serves as the cladding layer in place of thelayer 32. The thickness of the p-side nitride semiconductor layer 8 isreduced accordingly, so that the driving current decreases. The samelevel of properties as in Example 1 can be obtained except thatdescribed above.

Example 9

In Example 1, the composition of the fourth n-side nitride semiconductorlayers 18 a, 18 b is changed from GaN to AlGaN which contains a smalleramount of Al than the third n-side nitride semiconductor layer 16. Thenitride semiconductor laser is manufactured in the same manner as inExample 1 except for that described above. Approximately the same levelof properties as in Example 1 can be obtained.

Example 10

In Example 1, the entire fourth n-side nitride semiconductor layers 18a, 18 b are made undoped. The nitride semiconductor laser ismanufactured in the same manner as in Example 1 except that describedabove. The same level of properties as in Example 1 can be obtained.

Example 11

In Example 1, next, the temperature is set to 990° C., and the thicknessof the n-type Al_(0.09)Ga_(0.91)N layer doped with Si of 2×10¹⁸/cm³(third n-side nitride semiconductor layer 16) is set to 2 μm. Thenitride semiconductor laser is manufactured in the same manner as inExample 1 except for that described above. Amount of leakage of light tothe substrate side is reduced compared to that in Example 1 and theFFP-Y shape is improved. Approximately the same level of properties asin Example 1 can be obtained except that described above.

Example 12

In Example 1, GaN is used for the barrier layer 22 b and formed with atwo-layer structure of InGaN and GaN, with a smaller amount of In thanin the well layer. The nitride semiconductor laser is manufactured inthe same manner as in Example 1 except for that described above. Thedriving current decreases compared to that in Example 1. The same levelof properties as in Example 1 can be obtained except that describedabove.

Example 13

In Example 1, the well-cap layer is changed from GaN to AlInGaN. Thenitride semiconductor laser is manufactured in the same manner as inExample 1 except that described above. The same level of properties asin Example 1 can be obtained.

What is claimed is:
 1. A nitride semiconductor laser diode comprising: a substrate; an n-side nitride semiconductor layer containing an n-type impurity and disposed on the substrate; an active layer having a light emitting layer including In_(x)Al_(y)Ga_(1−x−y)N (0<x<1, 0≦y<1, and 0<x+y<1) and disposed on the n-side nitride semiconductor layer; and a p-side nitride semiconductor layer containing a p-type impurity and disposed on the active layer; wherein the lasing wavelength of the nitride semiconductor laser diode is 500 nm or greater, and a concentration distribution of the p-type impurity in a depth direction from the light emitting layer toward the surface of the p-side nitride semiconductor layer has a local maximum with the concentration of the p-type impurity of 5×10¹⁸ cm⁻³ or greater in a range within 300 nm from the top surface of the light emitting layer which is closest to the p-side nitride semiconductor layer, and after passing the local maximum, the concentration of the p-type impurity is not less than 6×10¹⁷ cm⁻³ in the range within 300 nm.
 2. The nitride semiconductor diode according to claim 1, wherein the local maximum is arranged within 150 nm from the top surface of the light emitting element which is closest to the p-side nitride semiconductor layer.
 3. The nitride semiconductor laser diode according to claim 1, wherein the concentration distribution of the p-type impurity in a depth direction reaches a local minimum of not less than 6×10¹⁷ cm⁻³ after passing the local maximum and in the range within 300 nm and, after passing the local minimum, increases to 1×10¹⁸ cm⁻³ or greater.
 4. The nitride semiconductor diode according to claim 3, wherein the local minimum is arranged within 250 nm from the top surface of the light emitting element which is closest to the p-side nitride semiconductor layer.
 5. A nitride semiconductor laser diode comprising; a substrate; an n-side nitride semiconductor layer containing an n-type impurity and disposed on the substrate; an active layer of a quantum well structure including a well layer including In_(x)Al_(y)Ga_(1−x−y)N (0<x<1, 0≦y<1, 0<x+y<1), and disposed on the n-side nitride semiconductor layer; and a p-side nitride semiconductor layer containing a p-type impurity and disposed on the active layer; wherein the n-side nitride semiconductor layer includes a GaN layer or AlGaN layer the well layer in the active layer has a lasing wavelength of 500 nm or greater, and wherein the nitride semiconductor laser diode comprises a first p-type nitride semiconductor layer in a range within 300 nm from the top surface of the well layer which is closest to the p-side nitride semiconductor layer, the first p-type nitride semiconductor layer being made of an Al-containing nitride semiconductor, having a bandgap larger than that of the well layer and having a p-type impurity concentration of 5×10¹⁸ cm⁻³ or greater and a second p-type semiconductor layer on the first p-type nitride semiconductor layer, the second p-type semiconductor layer having, within the 300 nm range a p-type impurity concentration smaller than a p-type impurity concentration in the first p-type semiconductor layer and not smaller than 6×10¹⁷ cm⁻³.
 6. The nitride semiconductor diode according to claim 5, wherein the first p-type nitride semiconductor layer is located within 150 nm from the top surface of the well layer which is closest to the p-side nitride semiconductor layer.
 7. The nitride semiconductor laser diode according to claim 5, wherein the concentration of the p-type impurity in the second p-type nitride semiconductor layer decreases with distance from an interference closer to the first p-type nitride semiconductor layer to a degree not less than 6×10¹⁷ cm⁻³, and then increases toward an interface of the opposite side.
 8. The nitride semiconductor diode according to claim 5, wherein the concentration of the p-type impurity in the second p-type nitride semiconductor layer has a local minimum within 250 nm from the top surface of the well layer which is closest to the p-side nitride semiconductor layer.
 9. The nitride semiconductor diode according to claim 5, wherein the concentration of p-type impurity in the second p-type nitride semiconductor layer is 1×10¹⁸ cm⁻³ or greater at a location of 300 nm from the top surface of the well layer which is closest to the p-side nitride semiconductor layer.
 10. The nitride semiconductor laser diode according to claim 5, having a third p-type nitride semiconductor layer on the second p-type nitride semiconductor layer, the third p-type nitride semiconductor layer having a band gap larger than the band gap of the second p-type nitride semiconductor layer and smaller than the band gap of the first p-type nitride semiconductor layer.
 11. A nitride semiconductor laser diode comprising: a substrate; an n-side nitride semiconductor layer containing an n-type impurity and disposed on the substrate; an active layer of a multi quantum well structure having a well layer including In_(x)Al_(y)Ga_(1−x−y)N (0<x<1, 0≦y<1, 0<x+y<1), and disposed on the n-side nitride semiconductor layer; and a p-side nitride semiconductor layer containing a p-type impurity and disposed on the active layer; wherein the n-side nitride semiconductor layer includes a GaN layer or AlGaN layer, the active layer includes a well layer having lasing wavelength of 500 nm or greater and a barrier layer located closest to the p-side in the active layer and having a thickness of 300 nm or less, the p-side nitride semiconductor layer includes, from the side closer to the active layer, a first p-type nitride semiconductor layer made of a nitride semiconductor containing Al having a band gap larger than the band gap of the well layer, having a p-type impurity with a concentration of 5×10¹⁸ cm⁻³ or greater, and having a thickness of 3 to 50 nm, a second p-type nitride semiconductor layer having a p-type impurity concentration smaller than a p-type impurity concentration of the first semiconductor layer and not smaller than 6×10¹⁷ cm⁻³, a third p-type nitride semiconductor layer having a band gap larger than the band gap of the second p-type nitride semiconductor layer and smaller than the band gap of the first p-type nitride semiconductor layer, and a fourth p-type nitride semiconductor layer having a p-type impurity concentration of 5×10¹⁹ cm⁻³ or greater; wherein dislocations originated in the well layer penetrate through the p-side nitride semiconductor layer.
 12. The nitride semiconductor laser diode according to claim 1, wherein the lasing wavelength is 560 nm or less.
 13. The nitride semiconductor laser diode according to claim 1, wherein the substrate is made of a nitride semiconductor having a dislocation density of 1×10⁷ cm⁻² or less.
 14. The nitride semiconductor laser diode according to claim 1 wherein the dislocation density of the substrate is lower than the dislocation density generated in the active layer.
 15. The nitride semiconductor laser diode according to claim 5 wherein the dislocation density of the substrate is lower than the dislocation density generated in the active layer.
 16. The nitride semiconductor laser diode according to claim 11 wherein the dislocation density of the substrate is lower than the dislocation density generated in the active layer.
 17. A display apparatus comprising a nitride semiconductor laser diode having a lasing wavelength of 500 to 560 nm according to claim 1, a nitride semiconductor laser diode having a lasing wavelength of 440 to 480 nm, and a nitride semiconductor laser diode having a lasing wavelength of 600 to 660 nm.
 18. The nitride semiconductor laser diode according to claim 1, wherein the substrate is made of a nitride semiconductor having a dislocation density of about 5×10⁵ cm⁻² or less.
 19. The nitride semiconductor laser diode according to claim 5, wherein the substrate is made of a nitride semiconductor having a dislocation density of about 5×10⁵ cm⁻² or less.
 20. The nitride semiconductor laser diode according to claim 11, wherein the substrate is made of a nitride semiconductor having a dislocation density of about 5×10⁵ cm ⁻² or less. 